The decision by Finland and Sweden to join the alliance has spurred debate about the move’s long-term consequences.

For seventy-seven years, Finland’s frontier with Russia has been peaceful. The border runs from the Baltic Sea through windswept farmlands and the Lapland wilderness to the frozen Arctic. It is, in places, just a farm fence designed to control wandering reindeer more than to thwart invading soldiers. Blue-and-white posts mark the Finnish side; red-and-green posts signify Russian soil. Both governments have encouraged cross-border tourism and economic ties to help “people to learn the basics of peaceful co-existence,” as the Finnish Ministry for Foreign Affairs says on its Web site. The Nordic nation resisted joining other Europeans in NATO; high-speed trains connect Helsinki to St. Petersburg. Since the Second World War, “Finlandization” has been synonymous with neutrality worldwide.

Then Vladimir Putin invaded Ukraine. In a whirlwind policy reversal, Finland announced, on Sunday, that it will seek membership in NATO. “This is a historic day,” President Sauli Niinistö said, at a press conference. “A new era begins.” On Tuesday, the Finnish Parliament voted 188–8 to join the alliance. If Finland is accepted, its eight-hundred-mile border will become NATO’s longest boundary with Russia, more than doubling the length of Europe’s front line. Sweden has followed suit. “We’re now facing a fundamentally changed security environment in Europe,” the Swedish Prime Minister, Magdalena Andersson, said, on Sunday. “The Kremlin has shown that they are prepared to use violence to achieve their political objectives and that they don’t hesitate to take enormous risks.”

The joint decision, three months into the war in Ukraine, reflects Europe’s fears about Putin’s long-term intentions—and the uncertain prospect of any real peaceful coexistence. For years, support within Finland for joining NATO had dipped to as low as twenty per cent. It jumped to fifty-three per cent in February, to sixty-two in March, and to a record high of seventy-six per cent this month, according to surveys conducted by Taloustutkimus for the Yle news agency. The leap is similar in Stockholm, where security doctrine has long avoided participating in military alliances. For the first time, the majority in Sweden, which has not been at war since the Napoleonic era, favor NATO membership.

NATO has embraced the two Northern European countries, which together form a strategic landmass. (Finland is about the size of Montana, and Sweden is slightly larger than California.) Rose Gottemoeller, a former deputy secretary-general of NATO and U.S. Under-Secretary of State for Arms Control and International Security, called it a “major strategic defeat for Russia, turning the Baltic Sea into a NATO lake.” The decision sends a powerful message that “aggression does not pay,” NATO’s secretary-general, Jens Stoltenberg, told reporters, over the weekend. “President Putin wants Ukraine defeated. NATO down. North America and Europe divided.” Instead, NATO is stronger than ever. And Europe and the United States are more united. Ukraine, he also boldly predicted, “can win this war.” On Sunday, NATO’s foreign ministers met with their Finnish and Swedish counterparts in Berlin. Afterward, U.S. Secretary of State Antony Blinken said there is “very strong consensus” for bringing Finland and Sweden into the alliance, despite a threat by Turkey to block them. The Biden Administration will host the leaders of Finland and Sweden and also their defense officials in Washington this week, while Blinken will meet with his Turkish counterpart at the U.N.

For the Nordic neighbors, the reversal may seem like a no-brainer. Putin “trolled us,” René Nyberg, a former Finnish Ambassador to Russia who later led a group promoting Finnish industry in Russia, told me. Putin’s duplicity—a “propaganda assault” invoking NATO as a pretext to seize Ukraine—“caused this enlargement,” he said. A detailed assessment by the Swedish foreign ministry concluded that Russia’s aggression reflected “a structural, long-term and significant deterioration of the security environment in Europe and globally.”

Yet the response by Finland and Sweden to what they view as an existential danger has also spawned one of the fiercest debates since the end of the Cold War about the world’s mightiest military alliance. One of NATO’s earliest critics was George F. Kennan, the architect of the U.S.’s “containment” strategy to isolate the Soviet Union. In an Op-Ed for the Times, in 1997, he warned that NATO expansion after the Soviets’ demise “would be the most fateful error of American policy in the entire post-cold-war era.” It could inflame nationalist, anti-Western, and militaristic tendencies in Russia, have an adverse effect on nascent Russian democracy, and hinder arms-control agreements. Today the debate is even more complicated.

For some, the way NATO agreed, in 1994, to welcome former Soviet allies “betrayed a catastrophic failure of imagination,” Daniel Treisman, a Russia expert at the University of California, Los Angeles, told me. The Czech Republic, Hungary, and Poland—three former Warsaw Pact members aligned with Moscow—joined in 1999. “The major international challenge of the nineteen-nineties was to integrate Russia securely into the Western world,” Treisman said. The West should have generated new financial, commercial, cultural, and political links—and new European security arrangements—to complement NATO. “If we had succeeded in that, the security of Eastern Europe would have taken care of itself,” he said. Instead, the West failed to understand how Moscow would perceive NATO’s guns edging eastward. Seven other nations, including three former Soviet republics and three more Warsaw Pact countries, became members in 2004. Discussion about adding Ukraine and Georgia, which began in 2008—long before either qualified for membership—also invited Putin “to call our bluff,” Treisman said. Four other countries joined between 2009 and 2020. Thirty nations, together, now have nearly four times more military personnel than Russia and also many more tanks, warplanes, and artillery. The Kremlin, however, has a larger arsenal of tactical nuclear weapons near Europe’s borders.

Even long-time supporters of U.S. and European security guarantees for Finland and Sweden are concerned about the consequences of the two northern nations joining the alliance. “Over all, Russia certainly loses here. But a weak and humiliated Russia is a dangerous Russia,” Anne-Marie Slaughter, a former director of policy planning at the State Department who is now the chief executive of the New America think tank, told me. She cited the history of a “weak and humiliated” Germany between the world wars that opened the way for Hitler’s rise to power and aggression across Europe. “Putin may well be able to stay in power for even longer on the strength of ‘the foreign enemy’ encroaching on Russia’s borders,” she said.

Slaughter added, “What is driving me crazy right now is the unspoken assumptions that are driving these choices, and that will once again block true pan-European security.” Taking tangible steps to support Ukraine, Finland, Sweden, and other European countries that legitimately feel threatened by Putin shouldn’t preclude attempts to further integrate Europe and Russia, which has been a major player on the Continent since 1648. Meanwhile, countries excluded from NATO “have less and less chance of ever being admitted to the charmed circle of ‘the West,’ and have less and less hope of being supported in their own struggles for decent democratic government,” Slaughter said.

Others, in a “realist” foreign-policy camp, believe that the United States should focus its clout, diplomacy, and resources on big-power rivalries and existential challenges. “The climate crisis is becoming an afterthought. China now takes a back seat to a vastly exaggerated Russian ‘threat,’ ” Andrew Bacevich, a West Point graduate and the president of the Quincy Institute, told me. Putin’s invasion has hijacked the U.S. national-security agenda, preëmpting a “much-needed debate about the wisdom of NATO expansion,” Bacevich said. “Passions take priority over strategy.”

The Swedish Peace and Arbitration Society has warned against escalating tensions with Russia in ways that could increase the threat of violent retaliation. “Joining NATO would be preparing for war,” Gabriella Irsten, the organization’s advocacy officer, told me. Indeed, Russia immediately vowed retaliation—“both of a military-technical and other nature”—to “neutralize” perceived threats from NATO expansion, the Russian foreign ministry said, on Friday. More ominously, Dmitry Polyanskiy, Russia’s deputy U.N. envoy, warned that, if Finland and Sweden “become part of the enemy, well, they bear all the risks.”

The curious irony is that, “for the longest time, Putin himself was at peace with the decision” to enlarge NATO, Gottemoeller, who is now at Stanford University, told me. In 2002, Putin signed the Rome Declaration, which created the NATO-Russia Council and its agenda of joint projects, such as containing nuclear proliferation and preventing drug smuggling from Afghanistan. Putin may exploit the perception of a European enemy because it helps him sustain power, Gottemoeller said. At the same time, she added, “it’s not a good long-term prognosis—Russia permanently at odds with its European neighbors, members of NATO and the E.U. or not.”

To ease the transition, Niinistö, the Finnish President, personally called Putin to explain the decision. “The surprise was that he took it so calmly,” Niinistö told CNN. “It seems that there are no immediate problems coming.” On Monday, Finland’s border with Russia was still quiet. “War in Ukraine has had very minor influences to the traffic,” Commander Kimmo Ahvonen, of the Finnish Border Guard, told me. “Border situation has been stable all the time, and coöperation with Russian authorities is working quite normally.”

The longer-term reality is a wider and deeper fissure dividing NATO and Russia. Europe is fractured, Alexander Stubb, the former Finnish Prime Minister, told CNN. A new Iron Curtain pits “an aggressive authoritarian, totalitarian revisionist and imperialist Russia” against dozens of European democracies working in tandem to isolate it. “That’s the future,” he said. Whatever the new sense of security is today in Finland and Sweden, every action generates a reaction—and further NATO expansion may well, too.